Someone can go do that literature review then.
On an anecdotal basis only, which sounds more plausible, then:
1) Murder rates are 4x lower chielfy/solely because of unleaded gas
or
2) Murder rates are 4x lower because of quite a few qualitative observations that we think data would confirm each played a role.
It's like saying, "I think murder rates are higher in Norway compared to Sweden not because of some big societal difference between the two countries, but probably because I remember a news story where a depraved lunatic shot 90 kids on an island."